Because of their mechanical properties and chemical stability, carbon films have been used as coating material for improving wear resistance and durability of a variety of parts.
In the prior art, films comprising carbon include diamond film, graphite film, amorphous carbon film, and the like. The manufacturing method and characteristics of these are as follows.
Diamond films are generally synthesized by filament CVD method, microwave plasma CVD method, or the like. The synthesis temperature is a high temperature of 700 degrees C. or greater. This synthesis is conducted by introducing approximately 1% of a hydrocarbon gas such as methane and a large amount (approximately 99%) of hydrogen gas. The reason for introducing hydrogen gas is to generate a large amount of atomic hydrogen. The atomic hydrogen reacts with and removes the amorphous component which is synthesized in the film.
The structure of the diamond film is a cubic crystal system, and a diffraction image reflecting a diamond structure is obtained with electron beam diffraction and X ray diffraction. With Raman spectroscopy analysis, there is a narrow peak near 1333 cm−1 corresponding to a diamond structure. Because of its crystalline quality, the film that is obtained has an extremely rough surface reflecting the crystalline structure. For its properties, it has a Knoop hardness of 9000 or greater and a density of 3.51 g/cm3 or greater.
On the hand, a graphite film is achieved by a vacuum deposition method or thermal decomposition of hydrocarbon gas. With the former method, synthesis occurs at a low temperature of 500 degrees C. or less, and with the latter method, synthesis is at a high temperature of 1000 degrees C. or greater. The crystal structure of graphite is a crystal with a hexagonal crystal system. It is very soft with a Knoop hardness of 200 or less. The density is approximately 2.25 g/cm3.
Amorphous carbon film is intermediate between diamond and graphite or between diamond and carbon resin.
The manufacturing method for amorphous carbon film includes plasma CVD method, ion vapor deposition method, sputter method and the like. All of these have in common a low synthesis temperature of 400 degrees C. or lower. They are generally synthesized at a temperature of approximately 200 degrees C. or less. In plasma CVD method or ion vapor deposition method, hydrocarbon gas is the raw material. In order to control the film quality, hydrogen gas is often added. In the sputter method, a rare gas such as argon is used for sputtering, and for control of film quality, hydrogen or hydrocarbon gas is generally added. Its structure, composition, and properties are as follows.
Its structure is amorphous and is thought to be a mixture of an sp3 structure reflecting a diamond structure, an sp2 structure reflecting a graphite structure, and bonding with hydrogen, and the like. With electron beam diffraction and X ray diffraction, a halo pattern reflecting the amorphous structure is obtained. Raman spectroscopy analysis shows a structure having a wide peak and shoulder near 1300-1600 cm−1 region. Because it is amorphous, the resulting film is smooth.
Amorphous carbon films typically contain 10-50 at % hydrogen. In Japanese Examined Patent Publication 5-58068, for example, there is one in which hydrogen content is 20-30 at %. Furthermore, in order to increase hardness, there have been proposals for decreasing hydrogen content to around several to 10 at %. In Japanese Laid-Open Patent Publication Number 3-158455 and Japanese Laid-Open Patent Publication Number 9-128708, there are disclosures of ones with hydrogen contents of several at %. Other than hydrogen, the addition of various elements has been attempted. There have been reports of examples in which metal, nitrogen, halogen atoms, and the like are added. In addition, with sputter methods in which solid carbon is used as a raw material, because film formation is conducted under a rare gas atmosphere of argon or the like, rare gas becomes incorporated in the film. Furthermore, in Japanese Laid-Open Patent Number 2000-80473, rare gas elements are aggressively incorporated in order to control internal stress, hardness, wear resistance, and the like.
With regard to the properties of amorphous carbon films, there is a wide range with a Knoop hardness of 1000-2000 and a density of 1.5-2.5 g/cm3. For example in Japanese Laid-Open Patent Publication Number 9-128708, one with a density of 1.5-2.2 g/cm3 has been shown.
In recent years, a variety of surface treatments have been used for tools, dies, machine parts, and the like. In order to achieve adhesion of the coating, various methods have been used. A typical method is one in which adhesion is improved by introducing an interlayer which has a high affinity to both the substrate and the coating. However, with a coating of an amorphous carbon film as described previously, adhesion is particularly difficult to achieve, and the following method has been proposed.
For example, in Japanese Patent Number 1444896, between vapor-deposited carbon and the substrate, there is an interlayer of a carbide, nitride, oxide, boride, or the like of a group 4a, 5a, or 6a metal of the periodic table.
In addition, in Japanese Patent Number 1940883, between a carbon hard film and a base metal, there is an interlayer having two layers with the lower layer being mainly Cr, Ti, and the upper layer being mainly Si or Ge. By further layering the interlayer, the first interlayer which is the lower layer has affinity with the substrate and the second interlayer which is the upper layer has affinity with the carbon film.
In Japanese Patent Number 2628595, there is a diamond carbon covering with an interlayer of a metal or alloy of Co, Cr, Ni, or the like. The recommended film thickness of the interlayer has a wide range of 10 nm-100 micrometers.
In Japanese Laid-Open Patent Number 8-232067, there is proposed a method of providing one or more layers of an interlayer which has a toughness greater than that of the hard carbon.
In addition, in Japanese Laid-Open Patent Number 11-49506, there is proposed a method for covering a substrate with a hard carbon film via an interlayer which contains a carbide of a metal of the substrate. By incorporating a component of the substrate in the interlayer, the affinity with the substrate is increased, and at the same time, by having a carbide as the interlayer, there is affinity with the carbon film.
Furthermore, in Japanese Laid Open Patent Publication Number 2000-87218, there is proposed a structure in which at the interface between an amorphous carbon film and a substrate, there is a mixed layer of thickness 10-500 angstroms containing a substrate-constituting element and an amorphous carbon film-constituting element, or else an interlayer of thickness 10-1000 angstroms is formed on the substrate surface, and between the interlayer and the amorphous carbon film, there is a mixed layer of thickness 10-500 angstroms containing an interlayer-constituting element and an amorphous carbon film-constituting element.
When amorphous carbon films are used in tools, dies, and machine parts, there are the following problems. In other words, hardness is inadequate, heat resistance is low, and adhesion is low.
As described above, amorphous carbon films generally have a Knoop hardness of approximately 1000-2000. In contrast with a diamond which has a Knoop hardness of approximately 9000 or greater, this is approximately 1/10 to ⅕ of the hardness. Especially when wear resistance is needed, materials with a higher hardness is desired. Although patents have disclosed ones with hardness of 2000 or greater, there have not been many amorphous carbon films with a hardness of 2000 or greater that have been put into practical use. This is thought to be because there is still the problem of adhesion which will be described later. Heat resistance of amorphous carbon film in atmospheric air is generally in the range of 350 to 450 degrees C. With higher temperatures, the hydrogen in the film is removed, oxidation proceeds, and hardness is dramatically reduced. Therefore, these amorphous carbon films can not be used in tools, dies, or machine parts with a high usage temperature.
With the biggest problem of adhesion, various methods as described previously have been used. However, even so, an adequate adhesion of amorphous carbon film is not achieved. The reason for this is thought to be as follows.
Firstly, when a carbide interlayer is used, there is difficulty in having a stable carbon concentration. With a slight change in synthesis conditions, adhesion with the substrate or lower layer becomes poor. The interface with the amorphous carbon film which is the upper layer also can readily become unstable. Film formation must be conducted while under extremely strict control.
Furthermore, nitrides, oxides, and borides do not have a very good affinity with amorphous carbon film. Therefore, formation of another interlayer on the upper layer becomes necessary, and there is an increase in the number of layers and interfaces, and there is increased instability.
Si and Ge are interlayer materials which have a comparatively stable adhesion with respect to amorphous carbon films. However, because of its brittleness, usage for high load purposes is problematic. In addition, they do not have good affinity with many metal substrates.
In addition, amorphous carbon film is a material with extremely high internal stress. In order to have the interlayer relieve some of the stress, a material with a high toughness such as metal or the like is introduced into the interlayer to have a stress relieving layer. However, fatigue accumulates in the area of this interlayer, and with time, this may result in peeling. This is the case when using an interlayer of a metal of Co, Cr, Ni, Mo.
With the method in which a mixed layer is provided between the substrate and amorphous carbon film or between the interlayer and amorphous carbon film, because the amorphous carbon film-constituting element is irradiated with high energy, at the time of mixed layer formation, the temperature of the irradiation surface increases, and the carbon in the mixed layer becomes graphite. The carbon component which becomes graphite has dramatically lower adhesion.
Furthermore, because amorphous carbon film is processed at a relatively low temperature, an adequate bonding at the interface is not obtained just by using a combination with a high affinity. Even when the interlayer contains substrate material, it is difficult to ensure adhesion when the processing temperature is low.
With regard to the above problems, there is the thought of using a diamond film instead of an amorphous carbon film. The hardness is a Knoop hardness of 9000 or greater, and heat resistance also surpasses that of amorphous carbon. Because the film formation temperature is high at 700 degrees C. or greater, if a suitable interlayer material is chosen, adhesion should theoretically be easily achieved. However, because the film formation temperature is so high, the materials that can be used become limited. More concretely, the materials are limited to ceramics or high hardness alloys. All-purpose and inexpensive materials such as iron materials and the like can not be used. In addition, because the surface roughness is high, for many uses such as tools, dies, machine parts, and the like, a diamond film can not be used without polishing.